This may gross you out so be forewarned.
We had leftovers from a Sunday before the 4th family get-together, but my wife wanted to save them for dinner. I needed something for lunch.
I spread natural peanut butter (the kind you gotta stir) on a slice of rye bread with caraway seeds. Then slices of processed cheese and provolone topped the peanut butter. Next thin slices of onion topped the cheese before putting on the other slice of rye bread.
I never made such a sandwich before, but I will make it again sometime.